Scanning electron microscopes (SEMs) direct a beam of electrons toward an object under observation and then detect secondary electrons emitted from the work piece. SEMs typically employ scintillator detectors for detecting secondary electrons emitted from the object being observed. When electrons impact material within the scintillator detector, the material emits a flash of light, that is, it scintillates. The flash of light is detected by a photon detector, such as a photomultiplier tube, which converts the light into an electric signal. Scintillator detectors are robust, stable and long lasting.
Focused ion beam (FIB) systems, on the other hand, direct a focused beam of ions toward a work piece. Because the ions in the beam are much more massive than the electrons in the primary beam of the SEM, the impact of the ions in the beam ejects from the work piece positive and negative ions, as well as electrons. To detect secondary particles, FIB systems typically use either a channel electron detector or a channel plate electron detector. By changing the electric potential of these detectors relative to the work piece, these detectors can be adjusted to detect either ions or electrons. The term “focused ion beam system” is used to also include a systems, commonly called a “dual beam systems,” which are typically equipped with both focused electron and focused ion sources.
Channel electron detectors typically comprise a curved tube with a cone at the entrance. The incoming ions or electrons strike the cone or the tube and cause the emission of secondary electrons, which in turn cause a cascade of additional electrons as the particles bounce off the sides on the curved tube. The electron current at the end of the tube is sufficiently great to be detected and amplified in a pre amplifier. Channel plate detectors comprise a thin plate having numerous small holes through the plate wherein each hole acts as a miniature channel electron multiplier or detector. Channel plates may be stacked for additional gain and to reduce ion feedback. The performance of channel electron detectors and channel plate detectors degrade over time. Also, the preamplifier for the channel electron multiplier or channel plate must be electrically floated, which makes the electronics more complex and prone to high voltage leakage currents generating detector output signal current instability.
When a channel plate detector is used in a FIB system, the detector is typically positioned below the objective lens of a FIB system and centered on the beam axis. The detector has a hole in the center to pass the primary ion beam and collects electrons or ions coaxially about the primary ion beam.
Mass spectrometers are instruments that determine the mass of particles. Mass spectrometers typically have two main components: a mass analyzer that separates particles according to their charge-to-mass ratio and a particle detector that detects the separated particles. In the mass analyzer, particles move through an electric field, a magnetic filed or combined magnetic and electric fields. The field separates the particles by altering their trajectories, depending upon their charge-to-mass ratio. The particle detector detects particles whose trajectories enter the detector. The detector has a small opening to accept a limited trajectory range in order to provide a high resolution of the charge-to-mass ratio. An ion detector in a FIB system, on the other hand, typically accepts particles over a large input angle.
Some mass spectrometers detect ions by using a structure that, when struck by ions, ejects electrons, which are then detected. Such a structure is referred to as an “ion-to-electron converter.” Generally speaking, these converters comprise a negatively charged, off-axis plate positioned to the side of the entering beam to be mass analyzed. The ions strike this plate to generate secondary electrons, which are detected by an electron multiplier.
U.S. Pat. No. 3,538,328 to Strausser describes such a detector that detects ions by generating electrons from the ion impact. Strausser describes a cone-shaped ion-to-electron converter placed around a narrow ion beam, with the wide side of the cone facing a scintillator electron detector. With the cone at a potential of between −0.1 kV and −10 kV, the positive ion beam enters the narrow end of the cone, the ions are reflected by the scintillator at ground potential and strike the inside surface of the cone. Secondary electrons produced at the cone surface by the ion impact are attracted to the scintillator. U.S. Pat. No. 4,101,771 to Hofer et al. builds upon the Strausser concept for ion detection by using a spherical surface facing a scintillator or semiconductor electron detector.
Ion-to-electron type detectors are not typically employed in focused ion beam systems because they cannot efficiently detect both electrons and ions. The efficiency of a detector is the ratio of the number of particles detected to the number of particles arriving at the detector.
Scintillators can be used to directly detect ions by maintaining the scintillator disk at a potential of about −10 kV to attract positive ions and convert their energy to photons. The ion-to-photon detection process is not very efficient, however. In addition, the impact of the ions on the scintillator significantly shortens the working life of the detector compared to that of a scintillator detector that detects primarily electrons.